The burgeoning field of endoscopic surgery has created a demand for a whole new set of articulating endoscopic surgical instruments for insertion through a trocar cannula. These instruments have evolved from similar instruments used in open surgery.
In open surgery, the working sections of retractors and other articulating instruments are made to specific non-linear shapes and sizes to conform to the specific organs or anatomical structures for which they were designed. In endoscopic surgery, these instruments often cannot fit through trocar cannulas. Therefore, a means must be employed wherein a substantially straightened instrument fits through a trocar cannula and articulates to a curvilinear or other non-linear shape after it is extended beyond the end of the cannula. Some prior art endoscopic devices achieve such articulation by means of linked elements in an articulating working section. Other prior art devices achieve such articulation by employing a single, deformable piece which forms at least part of the articulating working section.
The prior art includes variable retractors for endoscopic use wherein a retractable, deformable working section slides in and out of a sheath. When the working section is retracted, it is substantially straightened within the sheath and the sheath is extendable through a trocar cannula. After the sheath is extended through a trocar cannula, the working section is extended from the sheath and assumes a working, curvilinear configuration.
Typically, the deformable working section is a single curved strip which assumes a curved configuration in an unstressed state. The strip is elastically deformable from a curved configuration to a substantially straightened position. It is desirable to be able to bend the working section from a relatively small radius of curvature to a substantially straightened configuration.
The prior art also includes endoscopic instruments wherein an articulating section includes an articulating housing that contains a single, deformable strip. Typically, the strip assumes a substantially straightened configuration in an unstressed state. When the housing is articulated, the strip is elastically deformed to a curved configuration. It is desirable to be able to bend the strip to a relatively small radius of curvature.
The design and construction of an elastically deformable working section is limited by the properties of the materials used. Deformable working sections have been constructed from materials such as nitinol, which is a metal alloy composed of 50 percent nickel and 50 percent titanium and is relatively expensive.
The prior art further includes deformable working sections constructed from shape memory alloys. A description of these alloys and their use in the construction of medical instruments, including endoscopic surgical instruments, is discussed in U.S. Pat. Nos. 4,665,906 and 5,067,957. These shape memory alloys are also relatively expensive.
More conventional materials, such as spring steel, have less elasticity and therefore are less desirable for use in prior art designs of a deformable working section. For example, if desiring a portion or all of the working section to have a radius of curvature of approximately 5/16 of an inch using a spring temper stainless steel strip, the thickness of the strip is limited to no more than about 0.007 inches, which is the approximate maximum thickness of spring steel that will elastically bend between a straightened configuration and a 5/16 inch radius of curvature. Unfortunately a 0.007 inch thick strip of steel does not provide much stiffness or resistance to deflection while under load.
By comparison, a strip of nitinol having a thickness of 0.018 inches can elastically bend between a substantially straightened configuration and a 5/16 inch radius of curvature. The stiffness of a 0.018 inch thick nitinol strip is about five times greater than the stiffness of a 0.007 inch thick spring steel strip.
While using nitinol strengthens a deformable curved working section of an articulating endoscopic surgical instrument, its main drawback is its high cost. It would be desirable to construct the deformable working section from a low cost materials such as spring steel, having properties similar to that of nitinol. It would be desirable to develop variations of surgical instruments employing such a deformable working section to perform a variety of surgical functions.